1. Field
Various embodiments relate to a non-volatile memory device including a charge trapping layer and a method for fabricating the non-volatile memory device.
2. Description of the Related Art
Non-volatile semiconductor memory devices retain stored data even when their power supply is cut off. This is in contrast to volatile memory devices, which are unable to store data after power is cut off.
Non-volatile memory devices include memory cells, the basic units for storing data, which are arranged in columns and rows. Non-volatile memory devices may be organized depending on the type of memory cells they have.
One example of a non-volatile memory device is a floating gate-type NAND flash memory device (simply referred to as a flash memory device, hereafter), which has come into common use. Flash memory devices have a memory cell structure which includes a silicon oxide layer over a substrate, a floating gate silicon layer where charges are stored, a barrier oxide layer that serves as a barrier to prevent charge loss from the floating gate, and a control gate. The silicon oxide layer, the silicon layer, the barrier oxide layer, and the control gates are sequentially stacked, in that order.
The semiconductor industry continues to shrink flash memory cells to increase memory capacity as consumers continue to demand more and more data storage. Reducing cell size requires decreasing the height of floating gates, a part of the stacked structure.
Following the trend of more data storage, memory devices with a SONGS (Silicon-Oxide-Nitride-Oxide-Silicon) structure are being researched and developed. SONOS structure memory devices use a silicon nitride (e.g., Si3N4) as charge storage layer, instead of the traditional floating gate, and may retain data as well as effectively decrease memory cell height. Additionally, there is a modified SONOS structure, called a MONOS (Metal-Oxide-Nitride-Oxide-Silicon) structure. The charge storage structure and operating principles of the MONOS structure are the same as that of the SONOS structure, except that metal is used for the control gates, instead of silicon. The structure of the charge storage area is then silicon-oxide, silicon-nitride, silicon-oxide (Oxide-Nitride-Oxide: ONO). However, the function of the charge storage area remains the same.
A non-volatile memory device adopting the ONO structure uses a silicon nitride (e.g., Si3N4) for trapping charges in each unit cell. The principle behind this method is that the threshold voltage “Vth” is changed when charges are trapped in the silicon nitride.
Charge-trapping non-volatile memory devices do have a drawback in that data retention time is not long enough. This is due to insufficient density and/or insufficient uniformity of charge trapping sites in the silicon nitride (e.g., Si3N4) layer. To acquire sufficient storage capability, the silicon nitride (e.g., Si3N4) layer has to retain a certain thickness, which may interfere with the miniaturization process. This may also cause problems because a thick silicon nitride (e.g., Si3N4) layer may increase the required operating voltages and slow operating speeds. The inventive concept disclosed herein proposes a solution to these design difficulties.